Injection moulding is a cost effective process for forming articles of manufacture in large numbers.
Conventional injection moulding techniques may not be suitable for forming a moulded sheet article with a plurality of cross-sectional features (that is, a moulded sheet article that is other than a constant cross-sectional thickness throughout the entire article). One example of such an article is a mesh sheet with apertures between the members of the mesh. In cross section, there are a plurality of members separated by a plurality of apertures. In some applications, such mesh articles need relatively fine members that are subsequently tensioned. Conventional injection moulding techniques may not provide a mesh article with fine enough members to provide desired resilience.
Some materials are suitable for strain orientation. Examples of suitable materials include some of the HYTREL materials available from Du Pont. In an as-formed HYTREL article, the polymer chains in the material are relatively random. By stretching the article, the polymer chains become relatively aligned. That phenomenon is strain orientation. Strain orientation changes the material properties. Typically, the material becomes stronger and more elastic; that is the elastic limit is increased in comparison to the as-formed material. Additionally, the article generally lengthens in the direction of stretching and reduces in cross-section.
While some forms of HYTREL can be injection moulded to form products in which strain orientation can occur, it has been generally understood that HYTREL cannot be injection moulded to form a mesh article in which strain orientation can occur. If the recommended HYTREL injection moulding conditions of a melt temperature of about 225° C. and a mould temperature of between 30° C. and 40° C. are followed, the members of the formed mesh article will generally break rather than strain orienting a useful amount.
While laser cutting of a solid sheet can be used to form a mesh article with apertures between the members, that is a more expensive method for manufacturing a large number of products.
It is an object of at least preferred embodiments of the present invention to provide a method of injection moulding a polymeric resin to form an article which is suitable for strain orientation, or to at least provide the public with a useful choice.